The following is from the Nuance Support site:
"Version 9 and 10.0 cannot be installed on 64-bit Windows Vista operating systems and will prompt the user with an error message stating the operating system is not supported."
"Applications that are not supported on a 64-bit operating system can still be installed on systems using 64-bit processors if a 32-bit OS is used."
see: http://nuance.custhelp.com/app/answers/detail/a_id/5269/kw/64-bit
Version 9 is also listed as not supported by either 32 or 64-bit Windows 7 at: http://nuance.custhelp.com/app/answers/detail/a_id/5775
The following article however claims to show a method that it can be installed on Windows 7 64-bit. http://www.lehsys.com/2009/02/how-to-install-dragon-naturallyspeaking-9-on-windows-7-64-bit/